A microphone may pick up a sound around the microphone arranged in a space surrounded by a sound-reflecting material. In such a case, a sound may directly reach the microphone from a sound source and another sound emitted from the sound source may indirectly reach the microphone after being reflected off the material at least once. The sound indirectly reaching the microphone is referred to as a reverberation sound. The timing at which the reverberation sound reaches the microphone is later than the timing at which the sound directly reaches the microphone. If a reverberation sound is present, the reverberation sound is superimposed on the sound having directly reached the microphone, and the resulting sound is difficult to hear. Recently, a cellular phone having water-tightness has been available. If such a cellular phone is used in a bath room where reverberation is likely to take place, the sound picked up by the cellular phone becomes very hard to listen to because of reverberation. Techniques of reducing the reverberation sound have been studied.
In a reverberation removal method disclosed in Japanese Laid-open Patent Publication No. 2006-67127, an impulse response of a feedback path formed by an acoustic coupling between a speaker and a microphone is adaptively identified by an adaptive filter, and an echo component of the feedback path is estimated from a reverberation audio signal picked up by the microphone. In the reverberation removal method, the estimated echo component is subtracted from an output signal of the feedback path, and a filter coefficient of the adaptive filter is updated in a manner such that an estimated error of the estimated value of the echo component is minimized. Furthermore in the reverberation removal method, the filter coefficient with the estimated error of the estimated value of the echo component minimized is substituted for the impulse response of a reverberation space. An original audio signal is thus determined by performing a computation operation on a transfer function of the reverberation space determined from the filter coefficient, and the reverberation audio signal picked up by the microphone.
In the technique disclosed in Japanese Laid-open Patent Publication No. 2006-67127, the reverberation characteristics of the sound emitted by the speaker and picked up by the microphone are used to approximate the reverberation characteristics of a sound emitted from the sound source and reaching the microphone. A user may talk using a handsfree function of the cellular phone. In such a case, a distance between the sound source and the microphone may become much longer than or much shorter than a distance between the speaker and the microphone. The reverberation characteristics of the sound emitted by the speaker and picked up by the microphone become distinctly different from the reverberation characteristics of the sound emitted from the mouth of the user and reaching the microphone. As a result, the reverberation component of the sound emitted from the sound source is not appropriately removed.
Techniques of estimating a distance to a sound source using a microphone array have also been disclosed. For example, Japanese Laid-open Patent Publication No. 2011-53062 discloses a sound source distance measurement device. The sound source distance measurement device respectively converts signals received by a plurality of microphones into signals in a in frequency domain, and vectorizes the signals in the frequency domain to calculate a space correlation matrix. The sound source distance measurement device determines power of a direct sound and power of a reverberation sound from the space correlation matrix and location information of the microphones, references a distance direct-indirect ratio database, and determines a sound source distance estimation value corresponding to a direct to indirect ratio. The direct to indirect ratio is determined by dividing the direct sound power by the reverberation sound power.